This disclosure relates generally to the field of permeability determination of earthen formations. More specifically, the disclosure relates to probe type permeability determination in relatively unconsolidated formations.
Probe permeametry has proven to be a nondestructive, repeatable, and efficient method for measuring fine-scale variations in the permeability of earthen formations such as reservoir rocks. However, there are several technical challenges to the use of standard probe permeametry methods being directly applied to unconsolidated earthen formations. First, there is the problem of probe tip sealing force. When the probe tip seal is brought into contact with a formation sample surface, some axial force must be applied to the probe to compress the probe-tip seal and prevent fluid leakage at the interface between the probe tip and the formation sample. Unconsolidated samples lack the cohesion and strength required to resist the tip sealing force, which results in destructive compaction or displacement of rock grains when the probe tip comes into contact with the sample. Second, unconsolidated earthen formations are also difficult to prepare for probe permeametry, which often results in an uneven formation sample surface. An uneven formation sample surface may make it difficult to obtain a good seal between the probe tip and the formation sample.
Third, there are problems associated with the excavation and movement of small size (fine) rock grains within the formation sample. Even though fluid flowing through the formation samples during permeametry is only moderately pressurized, because such formation samples have high permeabilities the fluid flow rate can be high enough to overcome the cohesive and inertial forces in the formation sample and forcibly expel fine grains from the formation sample. Such movement of fine grains permanently alters the formation sample and results in permeability measurements that do not reflect the true permeability of the formation prior to structural alteration caused by mechanisms such as those described above.